Modern semiconductor processing frequently utilizes etchant removal of selected materials. For instance, a common insulative material in semiconductor circuitry is silicon nitride (Si.sub.3 N.sub.4), which is typically patterned into desired configurations by etching processes. A typical etching process for removing silicon nitride comprises exposing the silicon nitride to a liquid phosphoric acid solution. A phosphoric acid solution etch of silicon nitride has a particular advantage in that it is generally selective for silicon nitride relative to silicon dioxide. Accordingly, if both silicon nitride and silicon dioxide are exposed to the conditions of a phosphoric acid solution etch, the silicon nitride will be removed at a faster rate than will the silicon dioxide.
The reaction chemistry of a phosphoric acid solution etch of silicon nitride is as follows: EQU Si.sub.3 N.sub.4 +4H.sub.3 PO.sub.4 +10H.sub.2 O.fwdarw.Si.sub.3 O.sub.2 (OH).sub.8 +4NH.sub.4 H.sub.2 PO.sub.4
As can be seen from the above equation, water is a reactant that is consumed during etching of the silicon nitride with the phosphoric acid solution. The reaction conditions of the etch typically comprise a temperature of from about 150.degree. C. to about 170.degree. C., and typically comprise a pressure of about atmospheric pressure.
Under the typical etching conditions, a water concentration within the phosphoric acid solution can be reduced by water evaporation, as well as by water being consumed in the reaction process. Problems occur as a water concentration within the phosphoric acid solution decreases. For instance, a reaction rate can slow (or even stop) if water is not replenished. In an effort to overcome this problem, water (typically in the form of deionized water) is generally replaced at selected times, or selected temperature drifts, during a reaction process. Present methods for replacing deionized water have several associated problems, including: 1) it is difficult to accurately control etch rates; 2) the deionized water concentration in a reaction solution is not known, and accordingly a reaction rate can vary significantly from a beginning of a reaction to an end of the reaction; and 3) if an error occurs in a water replenishment mechanism and water is inadvertently not replenished at various points in a reaction process, the problem will not be detected until product wafers are removed and found to have an incomplete nitride strip.
For the above-discussed reasons, it would be desirable to develop alternative methods for maintaining a water concentration in a phosphoric acid solution during a nitride etch. More generally, it would be desirable to develop alternative methods for maintaining concentrations of selected species in solutions during semiconductor fabrication processes.